This invention relates to a method for preparing a pesticide microcapsule in which the release rate and release period of the pesticide can be controlled.
There are two major conventional methods to form a urea-formaldehyde resin membrane. In the first method, a water-soluble macromolecule is prepared with urea and formaldehyde. Thereafter, the macromolecule is microemulsified with oil-soluble components to form a microcapsule. See, for instance, U.S. Pat. Nos. 3,516,846 and 3,993,831 and Japanese Patent Publications 30282/71, 29783/70, 14739/69 and 18671/77. The method which is described in U.S. Pat. No. 3,516,846 and Japanese Patent 46-30282 is suitable for encapsulating a large amount of pesticide. In this procedure, a large amount of vegetable oil is used (the amount of the oil to that of aqueous prepolymer solution is 0.38-0.48:1). Japanese Patent 45-29483 discloses a method which uses a surface tension enhancer to thoroughly disperse an oil-soluble material in water-soluble encapsulating material, so as to encapsulate the oil-soluble material and act as a surface tension enhancer. Japanese Patent 44-14739 discloses a method that dissolves the nonpolar material in a hydrophobic liquid. Small droplets of the liquid are added to an aqueous water-soluble modified urea resin solution. After a crosslinking reaction, microcapsules suitable for pressure sensitive non-carbon copy paper are formed. The additions of the non-polar material by small droplets has disadvantages in large scale production of microcapsules. In Japanese Patent 52-18671, an oil-soluble solution of formaldehyde condensate is stirred with an aqueous solution of its polymer to form a water-in-oil emulsion. A reaction at the interface is carried out to form a microcapsule. The advantage of this method is the employment of water-soluble polymer as an emulsifier instead of using a surfactant. However, this method has the disadvantage of requiring additional preparation of an oil-soluble formaldehyde condensate. Japanese Patent Publication 62-149607 describes a method which uses 3-20% of surfactant to encapsulate dispersed ethoprophos in vegetable oil by using a urea-formaldehyde resin. A large amount of oil is used in this method (1-9 parts relative to the weight of aqueous prepolymer solution) and the amount of pesticide encapsulated is more than 3.0%.
The second method comprises the simultaneous formation of microcapsule and polymerization of urea and formaldehyde, as described in U.S. Pat. Nos. 4,001,140 and 4,089,802. Both of these patents suggest that simultaneous microencapsulation and polymerization can only be done through the use of carboxyl-substituted aliphatic hydrocarbon polycharged substances having negative charges. Therefore, application of this method is very limited. In addition, U.S. Pat. No. 4,353,809 uses polyvalent isocyanates to improve the emulsifiability. However, this method is complex and hard to control in its production process.